To graph the functions you can take some values and find the points that belongs to the lines.
Easily, we can take b=0 and find the u value, and then take u=0 and find the b value.
For equation 1: b+u=18
For equation 2: 2b+u=32
You can use that points to draw the lines.
If you graph the lines, you can see that they intersect in a point, that's mean the point satisfy the two equations, and it's the solution of the system.